An optical filter may be used to transmit a spectral band or a spectral component of incoming light. A high pass filter, for example, transmits light at wavelengths longer than an edge wavelength of the filter. Conversely, a low pass filter transmits light at wavelengths shorter than an edge wavelength. A bandpass filter transmits light at wavelengths proximate to a center wavelength of the filter within a bandwidth of the filter. A tunable bandpass filter is an optical filter, the center wavelength of which may be adjusted or tuned.
A spectrometer may measure an optical spectrum of incoming light. A scanning-type spectrometer may use one or more tunable bandpass filters to select different spectral components of the incoming light. A scanning-type spectrometer operates by scanning the center wavelength of the tunable bandpass filter while measuring optical power levels of light transmitted through the tunable bandpass filter, so as to obtain the optical spectrum. Alternatively, a polychromator-type spectrometer uses a wavelength-dispersing element optically coupled to a photodetector array for parallel detection of the optical spectrum.
Conventional optical filters and spectrometers are bulky, which limits their usefulness in portable light-sensing devices and applications. Linearly variable filters have been used in spectrometers to provide a wavelength separating function. Referring to FIG. 1A, a conventional linearly variable filter 10 may be illuminated with white light, which includes top 11, middle 12, and bottom 13 multi-wavelength light beams. The top 11, middle 12, and bottom 13 multi-wavelength light beams may strike the linearly variable filter 10 at respective top 11A, middle 12A, and bottom 13A locations. The linearly variable filter 10 may have a center wavelength of a passband varying linearly along an x-axis 18. For instance, the filter 10 may transmit a short wavelength peak 11B at the top location 11A; a middle wavelength peak 12B at the middle location 12A; and a long wavelength peak 13B at the bottom location 13A.
Referring to FIG. 1B, a conventional spectrometer 19 may include the linearly variable filter 10 of FIG. 1A, a tapered light pipe 14 disposed upstream of the linearly variable filter 10, and a linear array 15 of photodetectors disposed downstream of the linearly variable filter 10. In operation, non-collimated incoming light 16 may be conditioned by the light pipe 14 to produce a partially collimated light beam 17. The linearly variable filter 10 may transmit light at different wavelengths as explained above with reference to FIG. 1A. The tapered light pipe 14 may reduce a solid angle of the incoming light 16, thereby improving spectral selectivity of the linearly variable filter 10. The linear array 15 of photodetectors may detect optical power levels of light at different wavelengths, thereby obtaining an optical spectrum, not shown, of the incoming light 16.
The tapered light pipe 14 may often be the largest element of the spectrometer 19. A collimating element, such as tapered light pipe 14, may be needed because without it, the spectral selectivity of the linearly variable filter is degraded. This may happen because the linearly variable filter 10 includes a stack of thin dielectric films. The wavelength-selective properties of thin film filters are generally dependent on the angle of incidence of incoming light, which may deteriorate spectral selectivity and wavelength accuracy of thin film filters.